296 ANIMAL BIOLOGY 



might work at cross purposes, so that we would be unable 

 to accomplish anything that we wanted to do. 



In order to see how the nervous system acts we need 

 to gain some knowledge of how it is formed. Our nerves 

 are composed of bundles of nerve fibers. Each fiber 

 consists of (i) a central core of nervous substance, the 

 axis cylinder; (2) an external very thin sheath; and (3) 

 in most nerves a layer of fatty substance, the medullary 

 sheath, between the axis and cylinder and the outer cover- 

 ing. Each nerve fiber comes from a nerve, or ganglion cell, 

 and it terminates in various ways according to its function; 

 some nerves supply muscles (motor nerves), some end in 

 glands, and others (sensory nerves) end in some sense organ 

 and serve to carry sensory impulses to the central nervous 

 system. Nerve cells commonly have several branches 

 some of which may subdivide repeatedly. In the central 

 nervous system these branches meet and thus impulses 

 received by one cell may be conveyed to other cells and 

 finally be transmitted to remote parts of the body. Nerve 

 cells are frequently grouped together into masses called 

 ganglia which are found in various parts of our organism. 

 The large masses of nervous tissue making up the brain 

 and spinal cord have much the same composition as 

 ganglia, being composed of nerve cells and their branches, 

 together with blood and lymph vessels and a framework 

 of connective tissue binding the whole together. 



The nervous system is composed of two principal parts, 

 the cerebro-spinal system, and the sympathetic. The first 

 includes the brain, the spinal cord, and the nerves that 

 proceed from these central organs. Both brain and spinal 

 cord contain (i) gray matter which is composed largely 

 of nerve cells and their branching processes, and (2) white 

 matter which is formed of nerve fibers with few or no cells. 

 In the spinal cord the gray matter is central, forming a 



